Fuel injected internal combustion engines are well known. Fuel injection is a way of metering fuel into an internal combustion engine. Fuel delivery is typically through engine intake ports but is more recently directly into the cylinder through the engine head. Accordingly, fuel injection arrangements may be divided generally into multi-port fuel injection (MPFI), wherein fuel is injected into a runner of an air intake manifold ahead of a cylinder intake valve, and direct injection (DI), wherein fuel is injected directly into the combustion chamber of an engine cylinder, typically during or at the end of the compression stroke of the piston. DI is designed to allow greater control and precision of the fuel charge to the combustion chamber, providing the potential for better fuel economy and lower emissions. DI is also designed to allow higher compression ratios, providing the potential for delivering higher performance with lower fuel consumption compared to other fuel injection systems. As the industry moves more towards the fuel delivery directly into the cylinder, it is highly desirable in a modern internal combustion engine to provide high pressure fuel injectors that more precisely deliver fuel.
Generally, fuel injectors rely on internal valves to open a precise distance to deliver exact amounts of fuel to the engine. An electromagnetic fuel injector incorporates a solenoid armature, located between the pole piece of the solenoid and a fixed valve seat. The armature typically operates as a movable valve assembly. Electromagnetic fuel injectors are linear devices that meter fuel per electric pulse at a rate proportional to the width of the electric pulse. When an injector is energized, its movable valve assembly is lifted from one stop position against the force of a spring towards the opposite stop position. The distance between the stop positions constitutes the stroke.
A solenoid actuated fuel injector for automotive engines is required to operate with a small and precise stroke of its valve in order to provide a fuel flow rate within an established tolerance. The stroke of the moving mass of the fuel injector is critical to function, performance, and durability of the injector. Injectors for gasoline DI require a relatively high fuel pressure to operate. The fuel pressure may be, for example, as high as 1700 psi compared to about 60 psi required to operate a typical port fuel injection injector. Due to the higher operating pressure, the fuel flow of gasoline DI injectors is more sensitive to variations in stroke than port fuel injection injectors and, therefore, a tighter control of the stroke set is needed. Typically, a stroke tolerance of about +/−5 microns is desired for GDI injectors where a tolerance of about +/−14 microns is acceptable for port fuel injection injectors.
Methods for controlling the exactness of the valve opening are an ongoing design and manufacturing challenge. Current fuel injectors use a variety of methods to set and control the displacement of the valve. For example, adjusting the pole piece location is currently the most commonly used method for setting the stroke on fuel injectors. This method involves precisely pressing the pole piece to a position that gives the required valve displacement. Shortcomings of this method are the complexity of the part design, especially the achievement of the needed tolerances, and the process of accurately pressing the pole piece to the right depth without pressing too far. This approach also requires an external structure for the pole piece to slide inside thus adding more parts and cost. The sliding motion between the external structure and internal pole piece can also generate undesirable contamination in the injector. Stroke setting tolerance with this process can generally be in a +/−12 micron range.
Another current approach includes a threaded valve seat outer diameter and a threaded body inner diameter. By threading the outer diameter of the seat and the inner diameter of the body that the seat mates with, valve stroke is adjusted by controlling the depth that the seat is screwed into the body. This design is typically used on port injectors and functionally works satisfactory. The major shortcomings of this approach are the difficulty and cost of creating the very fine threads on the outer diameter of the small and hard seat as well as cutting threads on the inner diameter of the body. Once the correct stroke is set using this approach, the seat is typically spot welded to the body. An o-ring is usually fitted between the seat and the body to assure that no leakage occurs. Stroke setting tolerances with this process can generally be in a +/−12 micron range.
Still another approach is the selective flat shim method. The selection of a flat shim of a precise thickness to give the desired valve displacement is a long used method in high-pressure fuel injectors. The process typically involves taking interfacing component measurements, calculating the appropriate shim thickness, selecting the shim, and installing the shim into the injector during assembly. Shortcomings are that a large number of high precision shims of various thicknesses need to be on hand and ready for assembly. The mating part measurements are complex and difficult to integrate into a high volume manufacturing operation. Stroke setting tolerances with this process can generally be in a +/−5 micron range or better if disassembly and reassembly with a different shim is allowable. The shim selection method for setting the fuel injector stroke is, therefore, a very high cost process.
What is needed in the art is a simplified method for setting valve displacement in a fuel injector that involves fewer parts to be assembled, that involves parts that can be easily manufactured, and that can be easily integrated into a high volume manufacturing operation. It is a principal object of the present invention to provide a variable shim and valve seat assembly that enables a simplified method for setting the injector valve stroke.